Emergency break-in system for channel communication systems



@me Kw Mmmm w 55S ESl NlllxflJ OTY SS l .slh. www M ANIV. A Mmmm v 55mm e w fm T. N 1W HIS ATTRNEY June 6,l 1950 Patented June 6, 1950 EMERGENCY BREAK-IN SYSTEM FOR CHANNEL COMMUNICATION SYSTEMS Elbert W. Marlowe, Wilkinsburg, Paz, assigner to lThe Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application May 1s, 1948, serial No. 27,742

7 Claims. (Cl. 250;-13)

- My invention relates to communication systems of the type employing a single frequency channel for transmission and reception, and particularly to an improved system of the type described providing for an emergency break-in operation.

. When a communication system utilizes a single frequency channel for transmission and reception to which access may be had at any time by a stationdesiring to transmit, it will be apparent that another station is unable to communicate with the station which is transmitting. That is, communication systems of this type are usually operated on a push-to-talk basis, and during the time a station is transmitting its receiving equipment is rendered inactive, and vice versa. Thus a station which is transmitting cannot hear an incoming message since its receiver is cut off during the time the transmitting equipment is retained in its active condition.

' Such types of single channel communication systems are often used Where the receivers of all the stations of the system are retained active during non-communication periods ready to receive a message and a call sent out by any station is picked up by all oi the other stations. For example, such communication systems are used on aircraft, for railway train communications, etc. It is quite often necessary for a station to send out an emergency message that should be received without delay by all of the other stations, at least those stations within the usual range oi transmission. With the push-to-talk arrangement heretofore used, a calling station has no way to break in on a station whose transmitter is switched to an active condition and this station will not receive emergency messages. Y

,Accordingly an object of my invention is to provide an emergency calling system for single frequency channel communication systems.

, Another object of my invention is to provide an emergency calling system for single frequency channel communication systems in which the transmitter and receiver are alternately rendered active at unequal intervals at a sub-audible rate to provide recurrent transmission and reception.

A further object of my invention is to provide an emergency calling system for a single frequency channel communication system in which the transmitter and receiver of a particular station are alternatingly switched on and off for unequal intervals during such times as the station is transmitting.

Another object of my invention is to provide an emergency calling system for a single frequency channel communication system in which thev transmitter and receiver of a particular station are `alternately switched on and off for un-A equal intervals during such times as the station is transmitting, and the receiver of this station is provided with emergency call signal detecting means, responsive to an incoming emergency break-in signal, which causes the generation of a Warning signal which is supplied to the sound re producing means located at the station. Other objects of my invention and features ofA novelty therein will, be apparent from the following description taken in connection with the accompanyirrg drawing, v V

In practicing my invention, I provide at each station in a communication system employing a single 'frequency channel, means effective whenV the station is transmitting for recurrently deactivating the transmitter and activating the receiver for a relatively short time interval. Thus duringj transmission, intervals are provided during which` an incoming signal may be received, but these irltervals during which the transmission is cut oil` are short enough so that the brief interruptions.v Willrnot be detectedby the person listening at the receiving station, due to the phenomenon ofI persistence of hearing. Since the periodsduring which the receiver is active are comparatively short, an incoming signal from another station usually could not be heard at the transmitting station, especially if the equipment is located in noisy surroundings such as in the cab of a loco motive or in an airplane. Accordingly, I provicleV means vat each station for transmitting a distinc tive call signal, means for vdetecting such a call signal, and means governed by the call detecting means for generating a Warning tone which may be heard by the operator at the transmitting ,station in the loud-speaker or other sound repro-A ducer usually provided Vin such systems.

I shall describe one form of communication `system embodying my invention and shall then point out the novel features thereof in claims.

The accompanying drawing is a diagrammatic view showing a preferred embodiment of my invention. It is to be understood that although the apparatus is shown and described in connection with a space radio system, it is readily adaptable to carrier current communication or inductivecommunication systems.

Referring to the drawing, there is shown a staf,J

tion in a space radio communication system em-V ploying apparatus which embodies my invention,

it..being understood that the other stations of the system are provided with similar apparatus.-

The apparatus may be Vdivided into a receiver or receiving portion, a transmitter or transmitting portion including means for transmitting a distinctive call signal, switching means, and a call signal detector portion.

High voltage direct current energy is supplied to the apparatus from a source not shown, whose positive terminal is designated by the reference character-"BHO and the negative terminal of which is grounded. In order to clarify the drawing, the circuits for supplying energy to the heaters of the various electron tubes have not been shown.

The receiving portion of the equipment comprises a receiving antenna RA, demodulator amplifier DA, an output amplifier OA, and a loud speaker LS, and this portion of theA equipment is arranged in any of the well-known manners to provide reception of incoming signals.v ForV ex ample, the demodulator amplifier DA may be of the superheterodyne type, and it may be suitable for detecting any type of modulation, s uch as, for example, amplitude modulation or frequency modulation, depending on thesystem with which it' is used. High voltage direct current energy is supplied to the demodulator amplifier DA by a circuit including contact 9 of push button PB in its released position, so that the demodulator amplier DA is normally energized. The output amplier OA is'continuously energized, so that at this time incoming signals picked up by the receiving antenna RA are demodulated and amplied by thev demodulator ampliier DA, and supplied through the output amplifier OA to the loud-speaker LS, to thereby Yreproduce any-incoming signals.

The Ytransmitting portion of the equipment comprises a transmitter T, which supplies energy to a transmitting antenna TA, a call signal oscillator CSO, a microphone MIG, and a selector switch SL, The transmitter may be of any suitable type, and may bemodulated iny any manner, such as by frequency modulation or amplitude modulation depending on the system with which it is used, as previously 'explained in connection with the receiving equipment.v The transmitter is modulated by the microphone MIC, or by the callV signal oscillator CSO, as selected by a contact I3 of the selector switch SL. High voltage energy is supplied to the transmitter for the peration thereof by a circuit which includes a contact I of the selector switch SL in its lowermost position, or by a circuit including contact'lY of selector switch SL in its upper position,`and contact -Iy of the push button PB and which circuit connects the transmitter to an electronic switch ES, to be subsequently described, so that energy isisupplied therefrom to the transmitter T. YIt will be seen therefore, that the transmitter 'I1-is normally rendered inactive, while the receiver portion of the equipment is rendered active. A1-V though separate transmitting and receiving antenn'asare shown, it is to be understood that a single antenna may be employed for transmission and reception.

The switching portion of the equipment includes the push-to-talk button PB, and an electronic switch ES which may be of any suitable type, such as a multivibrator or other well known electronic switching circuits. It is deemed sucient to explain for the purposes of this disclosure that the electronic switch ES, which is continuously operative, is constructed and arranged to provide pulses of energy at recurrent intervals,

which may be supplied therefrom alternately to the receiver portionof the equipment orY the transmitter portion of the equipment, and that these pulses are of magnitude suicient to operate the receiver and transmitter. It is to be understood that means other than electronic switching means may be used, for example, a motor driven commutator arrangement may be substituted with equal facility. The equipment is arranged and constructed so that the electronic switch supplies pulses over the connection II which may be, for example, of the order of one milli-second in length, spaced at intervals of the order of one tenth of a second and energy is supplied over connection I 9 continuously except during the time in which the positive pulses are supplied over connection I1. yThat is, a short pulse of energy is supplied overconnection I'I at the same time that energy is removed for a like interval of time from the connection I9, so that when the push button PB isdepressed, energy is recurrently supplied to the demodulator amplifier DA for short intervals of time, and during these intervals, the supply of energy to the transmitter T over the connection I9 is interrupted.

The call signal detector portion of the equipment comprises an isolating amplier IA, a band,- pass filter BPF which is tuned'to the frequency of the call signal, oscillator CSO, a detector tube, VTI, a call signal lter CSF, aA pedestal tube VT4, a warning tone gate, comprising the vacuum tubesy VT2 and VT3, hereshown as being contained in a single envelope, and a warning,V tone oscillator WTO. It is believed that the call signal detector portion of the equipment can best be understood from a description of operation of the ap;- params.

In describing the operation ofthe equipmentY shown in the drawing, it will,Y rst be assumed that theY equipment is in its normal or receiving condition, as shown., At this time, the transmitter T is inactive, since nov energy is supplied thereto from the high Voltage source. Energy is supplied tothe demodulator amplifier DAi over contact`9-of push button PB in its released position,l

pa'ss lter BPF rejects normal speech frequenciesv and'thus the remainder of the callsignal detector equipment lis notoperated by ordinary incomingr speech modulated signals. At this time also the electronicswitch ES isl operating, but the contacts 9- and Tof pushv button PB'interrupt the circuits for controllingV thereceiver demodulator amplifier DA andthe transmitter T by the electronic switch.

If the operator desires to transmit a message,v

he depresses the push-to-talk button PB and speaks-into the microphone MIC. When the push buttonA PB-is depressed it cuts' oifthe continuous supply ofenergy to the demodulator amplifier DA, and establishes a connection overl contact 9 to supply energy from the electronic switch ESY to the demodulator amplifier DA. Additionally,

energy is s uppliedf-rom the electronic switch ESQ to the transmitter T- by the circuitV including they operated Contact I of the push button PB, and' Contact I5y of switchv SL in itsnormal position, The modulation produced by the operator speaking into the Vmicrophone MIC is supplied to the transmitter T over contact I3 of selector switch SL in its normal position, and as a result the transmitter is modulated by the voice frequency energy in accordance with the intelligence to be transmitted. As previously explained, the energy supplied to the transmitter T from the electronic switch ES over the circuit including the connection I9 is such that the transmitter is in operation except for extremely short intervals of time, so that the transmission of the transmitter 'I' may be considered as being recurrently interrupted for extremely short intervals. At the same time the energy supplied from the electronic switch ES over the connection I'I and contact 9 of -push button PB to the demodulator amplifier DA is such that the demodulator amplifier DA is inactive except for short intervals of time which coincide with the intervals of time during which the transmitter T is inactive. From the foregoing it will be seen that when the station is transmitting the transmitter and receiver are alternately switched on and off, the intervals during which the transmitter is switched 01T being extremely short as compared to the intervals it is switched on so that at a receiving station, the momentary interruption in the transmission of the sending station will not be noticeable by the person receiving the message, due to the persistence of hearing.

It will now be assumed that with a first station transmitting, a second station desires to break in on the transmission with an emergency message. Accordingly, the operator at the second station operates the selector switch SL of his station to its lowermost position. Steady high voltage energy is now supplied over contacts I and 2| of the selector switch SL to the transmitter T and to the call signal oscillator CSO, and the modulation circuit of the transmitter is shifted from the microphone MIC to the output of the call signal oscillator CSO by contact I3 of the selector switch SL. As a result, a continuous call signal is radiated from the transmitting antenna TA, having a modulation frequency determined by the call signal oscillator CSO, which may be, for example, 3000 cycles per second.

At the first station, during the intervals in which the transmitter is switched off and the receiver switched on, the radiated call signal transmitted from the second station is picked up by the receiving antenna RA and is amplified and demodulated by demodulator amplifier DA during its active periods. The resulting pulses of energy having a frequency equal to the call frequency signal are amplied and supplied to the loudspeaker LS to thereby produce short Ibursts of a, high pitched tone in the loud-speaker. 1 As previously explained, the station may be located in a noisy surrounding, such as a locomotive cab, so that it is possible that the brief high pitched signals might not be heard by the operator, especially if the gain control of the output amplifier was set for low output. Accordingly, the call signal detector of my invention provides for detecting the short pulses of the incoming emergency call signal and utilizes the signalpulses to produce a Warning tone which is supplied to the loud-speaker LS and is of sufficient volume and frequency to be readily noticed by the operator at the station. The demodulated and amplified call signal is supplied to the isolating amplifier IA by the connection in multiple with the output amplifier OA. This signal is amplified and passed to the band pass lter BPF, which is tuned to the frequency of the call signal oscil: lator in the case cited, for example, 3000 cycles per second. Accordingly, the 3000 cycle signal which is in the form of short pulses is passed to the detector tube VTI, where the signal is detected or rectified. The envelope of the rectifiedsignal is derived by the call signal filter CSF, here shown as a low pass filter, but which may be tuned to the repetition frequency of the incoming pulses. The resulting envelope is differentiated to produce positive and negative pulses by the differentiating network which includes resistors RI, R2 and condenser CI. The pulses resulting from the start of each envelope are negative and are passed by rectifier KI, here shown as an asymmetric unit, such as a copper oxide rectifier element, to the grid 3l of vacuum tube VT2, which with vacuum tube VT3 constitutes the warning tone gate. The warning tone gate is a type of electronic switch known as a univibrator, or so called one shot multivibrator, Well known in the art, which is characterized by having a stable state and an unstable state. The parts are so disposed that when a negative pulse is applied to the grid 3l of vacuum tube VT2, the univibrator changes from its stable state to its unstable state. The interval of time in Which it remains in its unstable state is determined by the circuit parameters, and after this predetermined interval, the univibrator reverts to its ,stable state, the period being determined by adjustment of the variable resistor R3. The output of the Warning tone gate is taken from the plate 33 of the tube VT2 and is a square positive impulse, initiated when a negative impulse is supplied to the grid 3| of the tube VT2, and termirated by the univibrator restoring to its normal S a, e.

The square positive output pulses from the warning tone gate are supplied to theiwarning tone oscillator WTO by the connection 3l. The warning tone oscillator WTO may take any form and it is deemed sufficient to explain that when the positive square output pulses are supplied thereto, a tone is generated by the Warning tone oscillator for the duration of the pulse. The audio frequency energy generated by the warning tone oscillator is supplied to the loudspeaker LS over the connections @l and 43, so that a warning tone is produced in the loudspeaker LS, to thereby call the attention of they operator at the first station to the fact that la second station desires to break in on his transmission.

When an emergency call signal is received at stations which are in the normal or receiving condition, the demodulator amplifier DA is continuously energized at this time, so that the call signal is reproduced as a continuous high pitched tone by the loud-speaker LS. However, to insure that the operator at the station in question will be warned of the emergency transmission, and to provide similarity of operation, the apparatus is arranged to provide a locally-generated audible warning signal in a manner similar to that previously described.

As previously explained, the electronic switch ES operates continuously and it is disposed for pulses'of positive polarity to be supplied therefrom to the grid i5 of the pedestal tube VT4. This tube is normally cut off by applying a positive bias potential to its Icathode 4l from the voltage divider constituted by resistors RI, R9 and RI3 connected between terminal B(I-) and ground. When an incoming emergency call sig-.,;

7;* nal is received, it is passed by the isolating am-VV plifier IA' and the band-passfilter' BPF to the de-` tector-tubefVT-I. Accordingly, the plate potential'of vacuum tube VTI falls andresults in a lowering of the potential of thecathodeof'the pedestal tube V'Iliso that tubeVTl 'isl render-'edrconductive each time that a positive vpulse-is sup-V pliedfrom the electronic switch to the grid'i. As aresult, the plate potential of the pedestal tube VT4 is pulsed negatively due to the inherent-1 phase shift ofthe tube, and these negative pulse.' a-re passed by the rectifier K2 to the grid 3i `on the warning tone gatetubeVTZ.

Each negative pulse trips the warning tone gate, and as a result ycauses the warning tone oscillator to supply relatively long pulses of audio frequency energy to the loud-speaker LS where the pulses are audibly reproduced to provide a warningsignal in the same manneras previously described.

The warning tone gate may have a period which varies over a Wide range of times, with a lower limit determined by the ability of the user to hear the warning tone adequately, and theupper limit being set just enough under the repetition interval of the triggering pulses to permitV the Vunivibrator to reset before the next triggering pulse is received. A preferred length of period might be of the order of one-half the repetition interval.

It is to be noted that the emergency calling signal is sent out as a continuous signal, rather than a series of interrupted transmissions; so that the vequipment at a station which is transmitting will be certain to pick up the signal.V If

the calling signal were to be sent out as a series of interrupted pulses controlled by the electronic switch ES, the .condition could occur in which the electronic switch at the station which wished to break in and the electronic switchat thefstation which was transmitting would be in synchronism in such a manner that the incoming emergency call signals would be present atthe time that the receiver was rendered inactive so that the incoming call signal 'would not be detected.

From the foregoing it will be seen that my invention provides a reliable emergency break-in system for a single frequency channel communication system which does not interfere with the` normal operation of the equipment, andv alsok provides for fast and reliable break-in operation;

Although I have herein shown and described only one form of communication system embodyng my invention, it is to be understood that various changes-and modifications may beV made therein within the scope of the appendedY claims without departing from the spirit and scope of my invention.

Havingy thus described my invention, what Iy claim is:

1. In a station for a communication system employing a single frequency channel, the combination comprising a transmitter for transmitting at said single frequency, said transmitter being arranged to be selectively modulated by the intelligence to be transmitted or modulated by a distinctive call signal frequency, a receiver conrprising a demodulator amplifier for demodulating and amplifying incoming signals, an output amplifier for amplifying the demodulatedsignals, manual control means for switchingrv between transmission and reception, continuouslyy operative. switching f means Y effective when saidA manual control means conditions the equipment for transmission `for recurrently deactivating-said transmitter and activating said receiver for a short time interval each of said intervals being relatively short compared'to the period between intervals, and call signal detector means operative-whena signal modulated by said call signal frequency is received for supplying distinctive audio frequency energy to said loud-speaker to thereby produce a distinctive call signal.

2. Ina station for a communication system employinga single frequency channel and comprising a transmitter arranged to lie-selectivelyv modulated by the intelligence to be transmitted or modulated by ak distinctive call signal fre-- quency, a receiver comprising a demodulator amplifier for demodulating and amplifying incoming signals, an output amplifier and a sound reproducer for audibly reproducing the incomingY signal, and manually controlled means for atwhich isv supplied directly to said sound reproducer independently of said output amplier,

and means including said lter for recurrentlyv tripping said second electronic switch in response to energy of said distinctive call frequency suppliedv from said filter, said warning tonevoscillator being governed by vsaid second electronici switch.

3. In a single frequency channel communication system stationhaving a transmitter and a receiver operableon said frequency channel on a push-to-talk 'basis by means of a pushto-talk to be'modulatedrby intelligence to ber transmitted and saidreceiver being arranged to reproduce the intelligence' received at a loud-speaker, the combination comprising, means to at times modulate said transmitter byY a given call signal frequency, automatic switching means, means governed by said push-to--talk7 means effective to recurrently deactivate said transmitter and activate saidreceiver Ifor. time` intervals which are short compared to the period between'intervals, and call signal detector means governed by;said,receiver and responsive to receivedl energy of saidgiven call signal frequency duringV said short time intervals tosupply to said loud-speaker audio frequencyv energy of distinctiveV type at recurrent intervals greater 78 than said short intervals.

and call signal detector means responsive to the reception of an incoming signalY a dis-- said call signal detec a second electronic switch having ay a period longer than said time intervals, a warning tone oscillator the outputof switch, said transmitter being arrangedv switch when set for, transmission to render said automatic switching'` 4. In a single frequency channel communication system station having a transmitter and a receiver operable on said frequency channel on a push-to-talk basis by means of a push-totalk switch, said transmitter being arranged to be modulated by intelligence to be transmitted and said receiver being arranged to reproduce the intelligence received at a loud-speaker, the combination comprising, means to at times modulate said transmitter by a given call signal frequency, automatic switching means, means governed by said push-to-tal switch when set for transmission to render said automatic switching means effective to recurrently deactivate said transmitter and activate said receiver for short time intervals which are short compared to the period between intervals, and a call signal detector means including a warning tone oscillator having connection to said receiver and responsive to energy of said given call signal frequency received during said short time intervals to actuate said warning tone oscillator at recurrent intervals, and said warning tone oscillator connected to said loud-speaker to sound a distinctive audible signal when said call signal frequency energy is received.

5. In a single frequency channel communication system station having a transmitter and a receiver operable on said frequency channel on a push-to-talk basis by means of a push-totalk switch. said transmitter being arranged to be modulated by intelligence to be transmitted and said receiver being arranged to reproduce the intelligence received at a loud speaker, the combination comprising, means to at times modulate said transmitter by a given call signal frequency, automatic switching means, means governed by said push-to-tal switch when set for transmission to render said automatic switching means effective to recurrently deactivate said transmitter and activate said receiver for short time intervals which are short compared to the period between intervals, a call signal detector means including a band-pass filter and a call signal lter connected to said receiver to receive energy therefrom, said band-pass lter to pass energy of said given frequency only, said call signal filter to pass energy having a frequency equal to the repetition of said short time intervals, and means including a warning tone oscillator controlled Yby said cali signal detector means and having connection to said loudspeaker to sound a warning signal when energy of said call signal frequency is received during said short time intervals.

6. In a single frequency channel communication system station having a transmitter 'and a receiver operable on said frequency channel on a push-to-talk basis by means of a push-totalk switch, said transmitter being arranged to be modulated by intelligence to be transmitted and said receiver being arranged to reproduce the intelligence received at a loud speaker, said "push-to-talk switch normally set to activate said receiver and deactivate said transmitter, the combination comprising a continuously active automatic switchingmeans operable at times to alternately activate and deactivate said receiver and transmitter in a cyclic manner, a selector switch operable independent of said pushto-talk switch and said automatic switching means to activate said transmitter and to modulate it by a given call signa1 frequency, a call signal detector means including a band-pass n1- ter connected to said receiver to receive energy therefrom, said band-pass lter to pass energy of said given frequency only, an electronic switch including a pair of electron tubes connected to have a stable and an unstable condition, means including another electron tube controlled jointly by said call detector means and said automatic switching means and having connection to said electronic switch to recurrently trip the electronic switch when energy of said call signal frequency is received with said push-to-tal switch at its normal position, and means including a tone oscillator controlled by said electronic switch and having connection to said loudspeaker to sound an audible signal.

7. In a single frequency channel communication system station having a transmitter and a receiver operable on said frequency channel on a push-to-talk basis by means of a push-totalk switch, said transmitter being arranged to be modulated by intelligence to be transmitted and said receiver being arranged to reproduce the intelligence received at a loud-speaker, the combination comprising, means to at times modulate said transmitter by a given call signal frequency, automatic switching means, means governed by said push-to-talk switch when set for transmission to render said automatic switching means effective to recurrently deactivate said transmitter and activate said receiver for time intervals which are short compared to the period between intervals, call signal detector means having connections to said receiver to receive a portion of the received energy and including a lter tuned to pass only energy of said call signal frequency, an electronic switch characterized by a stable and an unstable condition and having connection to said call signal detector means to trip it to its unstable condition in response to a pulse of said call signal energy received during an activated interval of said receiver, and a tone oscillator having connection to said loud-speaker and controlled by said electronic switch when tripped to its unstable condition to sound a warning signal.

ELBERT W. MARLOWE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,472,897 Murphy Nov. Y6, 1923 1,891,299 Anderson Dec. 20, 1932 2,369,230 Hansell Feb. 13, 1945 2,425,495 Trevor et al. Aug. 12, 1947 2,426,581 Atkins Sept. 2, 1947 2,431,167 Byrnes Nov. 18, 1947 

